1. Field of the Invention
The present invention relates generally to the field of internal combustion engine control systems and, in particular, to those systems that avoid the use of distributors and achieve efficient coil charging, even at higher engine speeds.
2. Description of the Prior Art
It is well-known that rotational forces are derived from internal combustion engines by the igniting of air/fuel (A/F) mixtures injected into cylinders of such engines, to impart rectilinear movement to pistons disposed within the cylinders, whereby rotational forces are imparted to a crankshaft. A spark plug is disposed within each cylinder and is electrically energized to create a spark igniting the A/F mixture. The spark is timed with respect to a top dead center (TDC) position of the crankshaft to cause burning of the A/F mixture to impart forces on the cylinder and, therefore, on the crankshaft at a point in time after the cylinder has reached its TDC position. The angular position of the rotatively driven crankshaft is typically measured with respect to the TDC position of the cylinder. In particular, the spark is generated at a point in time, i.e., spark instant (SI), corresponding to a selected angular position of the engine's crankshaft. Typically, the spark is generated at a position before the TDC position to ensure that the A/F mixture will be ignited and that the A/F mixture burning will produce maximum pressure within the cylinder at a point in time after the piston reaches its TDC position. The angular position of the crankshaft at SI is commonly known as the spark advance angle .theta.a and is measured in reference to the TDC position. Because the spark advance angle .theta.a directly effects when the burning of the A/F mixture takes place, the spark advance angle .theta.a also effects the amount of torque that will be delivered to the crankshaft. The relationship between the spark advance angle .theta.a and the crankshaft torque is a first order function and must be controlled precisely to obtain maximum fuel economy and to minimize the pollutants emitted by the engine.
The prior art has typically employed a switch or, more accurately, an array of mechanical switches rotatively coupled to the engine crankshaft and responsive to crankshaft rotation to close, thus, completing an electrical circuit to apply electrical energy selectively to the spark plugs. Such an array of switches is commonly known as a distributor. In early automobiles, the distributors were coupled to a hand operated lever mounted upon the steering column to manually advance or retard the spark instant. Mechanical governor-type distributors represented an improvement over the manually advanced controls, employing a centrifugal device coupled to the distributor to advance the spark instant automatically as a function of the crankshaft speed. Later, a manifold vacuum gauge was coupled to the internal combustion engine to sense the manifold vacuum and, thus, the load applied to the engine. Typically, such manifold vacuum devices were coupled to a mechanical diaphragm which served to retard the spark instant as the engine load increased and, thus, the manifold vacuum decreased.
Distributors of the prior art typically included a cam that is rotatively coupled by a reduction gear to the internal combustion engine and disposed to contact successively the contacts or points of the distributor, whereby the circuit to a corresponding spark plug is completed. Typically in the prior art, the physical position of the points and, thus, the spark advance angle .theta.a could be adjusted by the governor to vary the spark advance angle .theta.a as a function of crankshaft speed. Further, the prior art suggests that the manifold vacuum be sensed to position the points to retard the spark as a function of engine load.
Because mechanical distributors are limited as to angle .theta.a, the accuracy and the degree to which they may be controlled, electronic controls and, in particular, closed loop ignition systems have been employed to increase fuel efficiency and to decrease pollution emission. It is evident that the goals of decreasing pollution and increasing fuel efficiency are mutually exclusive in that as steps are taken to increase fuel efficiency, it becomes increasingly difficult to maintain the levels of pollution emission. Typically, emission control systems retard the spark advance angle .theta.a, thus, limiting pollution emission, but at the expense of good engine performance. In particular, the spark advance angle .theta.a is advanced as a nonlinear slope function of engine speed. The mechanical devices of the prior art, as well as many of the electronic controls, are able to implement such a function of spark advance angle .theta.a versus engine speed linearly, but with relatively poor accuracy and limited adjustment. As a result, engines with such controls cannot be accurately timed to meet the new, rigid standards imposed by the U.S. Government.
As described above, ignition control is effected by setting the ignition instant in terms of the spark advance angle .theta.a with respect to TDC. Typically of many systems is that disclosed in U.S. Pat. No. 4,015,566 of Walh, which includes an electronic ignition system for a four cycle internal combustion engine that controls the timing of the ignition instants with respect to the measured crankshaft position as a function of engine speed. In particular, the Walh system employs a transducer for providing a first train of pulses indicative of cam shaft position and a second train of pulses indicative of crankshaft speed. The first train of pulses is generated by a crankshaft position transducer coupled to the distributor shaft which is geared down by a ratio of 2 to 1 with respect to its crankshaft. For the four cylinder engine of Walh, the crankshaft position transducer outputs four pulses of the first train for each revolution of the distributor shaft and two pulses for each revolution of the engine crankshaft. In a typical four cylinder auto engine, there are four cycles, or cylinder firings, for each revolution of the distributor shaft and each set of two revolutions of the engine crankshaft. Thus, during the first revolution of the engine crankshaft, the ignition control will consecutively fire cylinders 1 and 3 and, during the second revolution of the engine crankshaft, the ignition control will fire successively cylinders 2 and 4 of the Walh engine.
The problem in achieving low pollution emission and efficient engine performance resides in the fact that prior art ignition control systems do not accurately measure and provide a high resolution signal indicative of the position of the engine crankshaft. For example, the Walh crankshaft position transducer generates only two output pulses for each revolution of its crankshaft. Thus, if the Walh engine accelerates rapidly, not only its output signal indicative of crankshaft speed, but also its signal indicative of crankshaft position are in error. To overcome these problems, Applicant discloses in his U.S. Pat. No. 4,494,509 entitled "HIGH RESOLUTION ELECTRONIC IGNITION CONTROL SYSTEM", a new and improved electronic ignition control which advances and retards SI with significantly improved accuracy or resolution with respect to the crankshaft position. Applicant hereby incorporates by reference the disclosure of his above identified patent into this application. In particular, Applicant's patent describes an optical encoder connected to the crankshaft of a distributor and comprising first and second encoder discs. The first encoder disc has a relatively large number of transmissive portions to generate a first, relatively high frequency signal, the frequency of which is an accurate indication of the angular rotational velocity of the engine crankshaft. The first train of pulses is applied to a phase locked loop which filters and outputs a signal of increased frequency proportional to that of the first train. The second encoder disc has a relatively few portions to generate a second train of signals of a second, lesser frequency. Each signal of the second train occurs in time when the crankshaft rotates past a fixed reference point in the rotation of the engine crankshaft. The reference point is set illustratively at 45.degree. before top dead center (BTDC). SI is accurately controlled to occur at the end of a variable length arc of crankshaft rotation starting at the 45.degree. BTDC reference point. The variable length of this arc is set dependent upon a selected engine parameter, e.g., the angular or rotational velocity of the engine crankshaft. The first high frequency train of signals is counted or integrated over a fixed period to obtain an accurate indication of crankshaft velocity. This accurate indication of crankshaft velocity is used to address one of a plurality of counts stored in memory. The counts are indicative of the degree or angle of advance or retard for that particular engine as a function of crankshaft velocity. A high resolution signal indicative of the crankshaft position is obtained by applying the high frequency signal of the phase locked loop to a crankshaft position counter, which initiates counting of the high frequency signal upon the occurrence of each signal of the second train. The crankshaft position counter counts to a point corresponding to the desired crankshaft arc as determined by the addressed count to provide an output signal, the occurrence of which controls SI. The SI is determined, not based upon a sensor which provides an output signal once or twice per revolution of the crankshaft, but rather upon the high frequency train of signals, thus, effecting ignition timing with a corresponding high degree of accuracy.
It is evident that fuel efficiency, pollution prevention and engine performance can be enhanced by improving the accuracy with which SI is set. In the ignition controlled system, as described in the above referenced U.S. Patent, a reduction gear mechanism included within the distributor couples the engine crankshaft to the first and second encoder discs, as described. Such a reduction gearing mechanism is a source of "backlash" which produces errors in the definition of the engine crankshaft position. In the ignition control systems of the prior art, the timing signals were of such low resolution that the inclusion of a reduction gearing, as typically employed between the engine crankshaft and the distributor, did not appreciably effect the timing of SI or the engine performance. However, as the accuracy of providing SI improves, the inaccuracies introduced by such reduction gearing are no longer acceptable.
To overcome the problems associated with the use of reduction gearing, Applicant discloses in his U.S. patent application Ser. No. 764,970, entitled "A PRECISION DISTRIBUTORLESS IGNITION CONTROL SYSTEM FOR INTERNAL COMBUSTION ENGINES", filed on Aug. 12, 1985, now U.S. Pat. No. 4,649,881, an electronic ignition system including a rotor directly coupled to the engine's crankshaft and comprising at least one first reference indicium and a plurality of N second reference indicia thereon for each first reference indicia. First and second signal generators are disposed at a point fixed in relation to the rotation of the crankshaft for providing first and second trains of signals in response respectively to the passage of each of the first and second reference indicia. SI is controlled by an arc termination circuit in the form of a counter, which is initiated in response to each first signal for measuring a variable crankshaft arc terminating at SI. The second train of signals is applied to a phase locked loop, which multiplies and outputs a signal of a relatively high frequency, which is applied to the aforementioned arc termination counter. In an illustrative embodiment described in this application, the rotor takes the form of a cylinder directly rotated by the motor's crankshaft and having a first plurality of slots therein corresponding to the first reference indicia and a second set of slots therein corresponding to the N second reference indicia. The first indicia are disposed on the rotor in a fixed relationship with the engine's crankshaft and, therefore, in a fixed relationship with TDC of the engine's cylinder. However, two signal generators are required to sense the first and second indicia, as well as circuitry for decoding and controlling which cylinder is to be fired next. The use of two or more signal generators adds to the cost of such ignition systems, as well as increases the difficulty of synchronizing the high resolution crankshaft signals as derived from the second signal generator.
It would be desirable to employ but a single signal generator that would not only supply a train of high resolution signals or pulses, but also a signal indicative of when the crankshaft rotates past a cylinder reference point such as TDC. The problem of using a single signal generator lies in the ambiguity presented by a single train of high resolution signals particularly when that ignition system must accurately control SI over an extremely wide speed range of the crankshaft varying from low engine speeds below 30 ERPM observed during engine cranking, to high speeds of over 12,000 ERPM required for racing engines. This is a speed range of 400:1. In addition, the rotation of the crankshaft is not constant; it may stop and then immediately start again. Further, rapid acceleration and deceleration is often imposed on the engine's crankshaft.
Further, an electronic ignition system must not only control SI, but also effect ignitions within a predetermined sequence of the cylinders. If a cylinder is fired out of turn, combustion could take place within a cylinder with its intake valve open, thus possibly causing an engine fire. Thus, it is desired to provide a high resolution signal that is capable of giving very accurate indication of crankshaft position, while ensuring that reference data indicative of the relative position of the crankshaft to the engine cylinders is provided so that synchronization is achieved within one resolution of the crankshaft over a very large speed range.
The above-identified patent of applicant further describes a circuit for charging the ignition coils with sufficient current, regardless of crankshaft speed. Charging the ignition coils with sufficient current is vital to produce the required spark energy for the engine's spark plugs. If the coil charging time is too long, power is wasted and the switching devices employed to selectively apply current to the coils, over-heated. In turn, switches, e.g., power transistors, of increased rating and cost must be employed in such systems.
On the other hand, if the coil charging time is to short, the ignition coil will not be sufficiently charged and the coil energy applied to the spark plugs may be insufficient to fire the A/F mixture. It is further realized that battery condition and engine speed may unduly effect the coil charging time. If timing is derived from a sensor connected to the engine's crankshaft and coil cut on time is derived as a function of the crankshaft position, the coil charging time will decrease as engine speed increases. This effect is particularly true for six and eight cylinder engines, because of the increased number of ignitions per crankshaft revolution required for such engine configurations. Further, as a battery grows older, its current output decreases, which may effect the current charging applied to the ignition coil.